As known in the prior art, .sup.238 PuO.sub.2 normally contains amounts of .sup.17 O and .sup.18 O isotopes in addition to the content of .sup.16 O and the presence of these other oxygen isotopes produces an unfavorable neutron emission ratio resulting from alpha-neutron reactions thereby rendering such material potentially injurious to health. .sup.238 PuO.sub.2 is generally available today containing 80% or 90% .sup.238 Pu with the balance containing other isotopes of plutonium but primarily .sup.238 Pu. It has already been suggested by the art, and in particular in U.S. Pat. No. 3,515,875 to V. Keshishian that .sup.238 PuO.sub.2 could be substantially improved through the exchange of the .sup.17 O and the .sup.18 O isotopes by the .sup.16 O isotope. Although several processes have been suggested by the art whereby this exchange by .sup.16 O may be carried out, such processes, as have been previously advanced, suffer from the admitted disadvantages and drawbacks of low exchange rates, low overall process efficiency and most important high cost to carry out the required exchange.